1. Field of the Invention
This invention relates to silver imaged aluminum plates such as printing plates and to acid solutions useful for deleting these silver images from the plate surface.
2. Description of the Prior Art
Silver imaged aluminum printing plates are well known in the art as evidenced by the well known silver halide diffusion transfer process for making printing plates. The capability of deleting unwanted silver image areas is very important due to the time, materials, and labor savings in not having to make a new printing plate to replace plates containing unwanted silver images. This capability is even more important if the imperfection is not discovered until the plate is on the printing press due to the very high cost of press time especially on the large presses. Due to the very small amount of silver present in the image areas of the silver halide diffusion transfer plates, i.e., less than 1 gram of silver per square meter, any unwanted silver image portions can readily be removed by simple mechanical means such as use of an ordinary rubber pencil eraser. However, in a process disclosed in copending applications U.S. Ser. No. 54,627, filed July 13, 1970 now U.S. Pat. No. 3,807,304 and U.S. Ser. No. 320,426, filed Jan. 2, 1973, abandoned, each of which is incorporated herein by reference, a printing process is described wherein the silver imaged printing plate is produced having run length capability considerably in excess of that previously capable of being produced by the silver halide diffusion transfer printing plate mentioned previously. This longer run length printing plate contains coherent silver images adherently and intimately bonded to an aluminum support which is preferably grained and anodized. The grain of the support is preferably at least about 1 micron in depth and may be produced by any of the common techniques known to the art such as brush graining, chemical graining, or sand blasting. The silver in the areas of maximum density of these new long run printing plates contain in excess of 1 gram of silver per square meter and preferably between about 3 and about 12 grams of silver per square meter. Preferably the anodization has been carried out to produce a barrier oxide layer at least about 100 A in thickness.
Attempts to utilize ordinary mechanical means for deleting unwanted silver images from these new silver imaged printing plates has proved unsuccessful. Using a rubber pencil eraser either takes too long or removal of the silver image is not possible. Using more abrasive mechanical eraser means suffers from the problem of either being too slow, image removal is incomplete because some of the silver is still present in the grain of the plate or the oxide layer on the aluminum plates is removed thereby exposing bare aluminum requiring extra steps to convert the more oleophilic exposed bare aluminum areas to hydrophilic areas. Ordinary acid and alkali deletion fluids known to the art are either of inadequate strength to remove the silver as rapidly as desired, i.e., in a time period of 2 or 3 minutes or less, or the solution such as that of sodium hydroxide was so strong that it would remove the aluminum oxide layer thus exposing bare aluminum and creating problems as mentioned above.
Additionally, any method of deleting silver images should be useful when the silver imaged aluminum support is in a vertical position since often the silver imaged printing plate is already in position on the printing press and it is most convenient to delete unwanted images while the plate is still on the plate drum. Access to the plate may be most convenient when the plate is in a vertical or other irregular position besides facing upward in a horizontal position. Thus a conventional liquid chemical deletion fluid is not satisfactory for deleting images from plates in these irregular positions since it runs off from the image to be deleted thus resulting in incomplete and non-selective deletion.